Photoresists become transparent

Materials consisting of both polymers and inorganic particles have been of interest for several decades. These materials possess the ease of processing of polymer substrates along with the integrated benefits of the inorganic phase such as magnetism, conductivity, or luminescence.

 

Now scientists in the United States have successfully produced uniformly transparent magnetic photoresists for microdevices requiring high transparency as well as some magnetic properties. [Gach et al., Biomaterials (2010), 31, 8810].

 

Microfabricated devices possessing magnetic properties are of great utility in bioanalytical microdevices due to their controlled manipulation with external magnets. Current methods for creating magnetic microdevices yield a low-transparency material preventing light microscopy-based inspection of biological specimens on the structures. Uniformly transparent magnetic photoresists were developed for microdevices that require high transparency as well as consistent magnetism across the structure. Colloidal formation of 10 nm maghemite particles was minimized during addition to the negative photoresists SU-8 and 1002F through organic capping of the nanoparticles and utilization of solvent-based dispersion techniques. Photoresists with maghemite concentrations of 0.01–1% had a high transparency due to the even dispersal of maghemite nanoparticles within the polymer as observed with transmission electron microscopy (TEM). These magnetic photoresists were used to fabricate microstructures with aspect ratios up to 4:1 and a resolution of 3 μm. Various cell lines showed excellent adhesion and viability on the magnetic photoresists.

 

Inspection of cells cultured on the magnetic photoresists with TEM showed cellular uptake of magnetic nanoparticles leeched from the photoresists. Cellular contamination by magnetic nanoparticles was eliminated by capping the magnetic photoresist surface with native 1002F photoresist or by removing the top layer of the magnetic photoresist through surface roughening.

 

The utility of these magnetic photoresists was demonstrated by sorting single cells (HeLa, RBL and 3T3 cells) cultured on arrays of releasable magnetic micropallets. 100% of magnetic micropallets with attached cells were collected following release from the array. 85–92% of the collected cells expanded into colonies

 

These polymeric magnetic materials should find wide use in the fabrication of structures for BioMEMS applications such as magnetic cell arrays, micro actuators, magnetic cantilevers, magnetic AFM probes, stir bars, sensors, relays and magneto-optical devices.